A hydrostatic power transmission which uses a pair of hydraulic pump/motors and is employed as a continuously variable transmission has been known. The conventional transmission has the advantage that speed can be continuously varied. However, when it is used to drive a device that can be run with low torque at high speeds, such as a winder or centrifugal separator, both of the hydraulic pump/motors are required to have variable displacement; otherwise, efficient operation would not be attained. A hydrostatic power transmission used to drive such a device is shown in FIG. 3, where the hydraulic pump/motors p and q, of variable displacement type, re connected in series with each other via a hydraulic circuit r. (A booster circuit and other circuits are omitted in the figure.) For example, power from a prime mover is applied to hydraulic pump/motor p, and the power delivered from the other pump/motor q can be transmitted to the rotor of a centrifugal separator, the drive wheels of an automobile, or the like. The transmission is controlled as shown in FIG. 4 from a start to a stationary operation at a high-speed in order to improve the efficiency of the system as much as possible.
More specifically, when it is started, the displacement of the pump/motor q on the output side is fixed at its maximum value, while the displacement of the other pump/motor p on the input side is made to be zero. Then, the displacement of the pump/motor p acting as a pump on the input side is gradually increased. Thus, the hydraulic pump/motor q functioning as a motor on the output side, increases the rotational speed while maintaining a high torque. When the displacement of the pump/motor p on the input side reaches the maximum, the control over the displacement of the pump/motor p ceases. Then, the displacement of the pump/motor q on the output side is gradually reduced. Thus, the rotational speed of the pump/motor q on the output side is increased while the torque is decreased until stationary operation at a certain high-speed is reached. That is, the transmission is operated with a constant torque at low speeds until the displacement of the pump/motor p on the input side reaches its maximum value. Thereafter, the displacement of the pump/motor q on the output side is controlled, and the transmission is operated with a constant horsepower at high speeds. Therefore, the transmission can operate efficiently, taking full advantage of its characteristics.
In general, hydraulic pump/motors of a variable displacement type have many, accurately manufactured movable parts and are very complex in structure. Therefore, where such hydraulic pump/motors are both used, the whole structure of the transmission is complex and expensive to manufacture. Since the displacements of the pump/motors must be separately controlled, the control system is made more complex, and the cost is necessarily increased. Another disadvantage arises from the fact that hydraulic pump/motors of the variable displacement type leak somewhat more working fluid than the pump/motors of fixed displacement type, because the former type incorporates more movable seals. Therefore, the variable-displacement type tends to produce slightly larger energy loss than the fixed-displacement type. This might adversely affect the efficiency of the whole system.